This invention relates to addition-curable silicone compositions. More particularly, this invention relates to addition-curable silicone compositions which are self-bonding to various substrates, such as plastics, metals, and glass.
Addition-curable silicone compositions may be applied to various substrates for various purposes. For example, such compositions may be applied to various synthetic fiber substrates to impart enhanced physical properties to the resulting laminate so that the laminate can be used for such purposes as gaskets, radiator hoses and the like. Further, for various hard substrates, such as metals and glass, so as to impart improved weatherability to the resulting laminated product.
Sometimes, it is necessary to apply addition-curable silicone compositions to heat sensitive substrates, e.g., polyolefins. Coating compositions which require a high temperature in order to cure are generally not compatible with heat sensitive substrates. Therefore, it would be further desirable to provide an addition-curable silicone composition which will cure at lower temperatures in a reasonable period of time, e.g. about 2-3 hours.
Presently, when an addition curable silicone composition is applied to various substrates, it is often necessary to use a primer, that is, a primer is applied on the surface of the substrate and then the silicone rubber composition is applied thereover and cured onto the substrate. In many cases, if a silicone rubber composition is applied to a substrate without a primer, the bond between the silicone rubber and the substrate is not as strong as would be desired. In discussing bonds, it is necessary to distinguish between two types of failure that may occur in the bonding between a silicone rubber layer and a substrate, that is, cohesive failure and adhesive failure. Insofar as it relates to this application, adhesive failure occurs when the silicone rubber layer separates from the substrate at the point wherein the two layers meet, that is, the bond between the silicone rubber layer and the substrate ruptures before the silicone rubber layer or the substrate. Cohesive failure occurs when the silicone rubber layer or the substrate rupture before the bond between the silicone rubber layer and the substrate fails.
The use of a primer entails an additional step in the preparation of the laminate and thus is costly as well as time consuming. Therefore, it would be desirable to provide an addition-curable silicone composition which does not require a primer to be applied to the surface of the substrate and then the silicone composition applied over the primer.
The present invention is directed to solventless, addition-curable silicone compositions which combine a critical SiH:SiVinyl molar ratio with an adhesion promoter selected from certain silylfumarates, silylmaleates, or silylmaleimide compounds and which cure at relatively low temperatures in a reasonable length of time to form superior adhesives having excellent physical and lap shear adhesive properties and which will adhere to various substrates without the use of a primer.
The use of silylfumarate and silylmaleate compounds as adhesion promoters in silicone compositions is known in the art. Reference is made, for example, to U.S. Pat. Nos. 3,941,741 (De Zuba et al.), which discloses the use of a maleate, fumarate, silylmaleate, or silylfumarate compound as a self-bonding additive in a heat-curable silicone rubber composition. Reference is further made to 4,273,698 (Smith, Jr. et al.); 4,308,372 (Smith, Jr. et al.); and 4,395,507 (Dziark et al.), which disclose the use of a silyl maleate, a silyl fumarate, or a silyl succinate as an adhesion promoter in room temperature curable silicone compositions.
The use of silylfumarate and silylmaleate compounds as adhesion promoters in addition-curable silicone compositions is also known in the art. Reference is made, for example, to U.S. Pat. No. 4,891,407 (Mitchell).
U.S. Pat. No. 4,891,407 to Mitchell is directed to a composition having interpenetrating matrices containing two polymer networks. The first is any polymer that is capable of being stretched, drawn, or expanded so as to obtain a microstructure characterized by nodes interconnected by very small fibrils and is preferably polytetrafluoroethylene. The second polymer network can be an addition curable silicone composition containing an alkenyl-containing polydiorganosiloxane, an organohydrogenpolysiloxane, an organic peroxide or a precious metal-containing catalyst, and, optionally, a reinforcing organopolysiloxane resin, extending and/or reinforcing fillers. The addition-curable composition preferably further contains an adhesion promoter which Mitchell discloses can be any of the maleate and fumarate-functional silanes disclosed in U.S. Pat. No. 3,759,968 to Berger et al., which can be a silylmaleate or silylfumarate, such as bis[3-(trimethoxysilyl)-propyl]maleate or bis[3-(trimethoxysilyl)propyl]fumarate. The cure temperature disclosed in Mitchell is in the range of 200.degree.-300.degree. C. at a cure time of about 2 hours (see column 6, lines 6-7, and column 8, line 57). It is further disclosed in Mitchell that in a preferred embodiment of the invention thereof, the addition curable silicone composition will contain an inhibitor which will prevent cure below 100.degree. C. (see column 5, lines 55-58).
The present invention is based on the discovery that an addition curable silicone composition containing an adhesion promoter selected from certain silylfumarates, silylmaleates, or silylmaleimide compounds in combination with critical molar ratios of SiH:SiVinyl will cure at lower temperatures, i.e., about 100.degree. C. to about 150.degree. C. in a reasonable period of time, i.e., from about 15 minutes to about 2 hours, to form an adhesive having excellent physical properties and excellent lap shear adhesive properties.